Over the last few years, RNA interference (RNAi) has emerged as an effective method of silencing gene expression in a variety of organisms, particularly mammals (19). Among its many applications are the characterization and regulation of gene function, analysis of signaling pathway and target validation. Another intriguing aspect of RNAi is its potential therapeutic value. The RNAi response in mammalian cells mediated by dsRNA is a well-defined two-step process. Initially, the dsRNA is cleaved into small interfering RNAs (siRNA) of approximately 19 to 25 nucleotide (nt) by an RNase III-like enzyme known as Dicer. Then, the siRNA is incorporated into a RNA-induced silencing complex (RISC), which destroys mRNAs that are homologous to the integral siRNA (45). In mammalian cells, interferon-mediated antiviral response to long dsRNA (>30 bp) causes the global shutdown of protein synthesis. To bypass this non-specific effect, small siRNA (<30 nt) has been used to induce reliable and efficient knockdown of target genes while evading the interferon response (13).
Gene silencing can be induced by direct transfection of cells with chemically synthesized (13) or in vitro transcribed siRNA (24, 30, 33). Alternatively, it can be obtained by transfecting a plasmid or transducing a viral vector encoding a short hairpin RNA (shRNA) driven by a RNA polymerase (pol) III promoter, including U6, H1, 7SK and tRNA promoters (5, 15, 38, 43), or a pol II promoter such as CMV or SP-C (16, 42). shRNAs consist of short inverted repeats separated by a small loop sequence and is rapidly processed by the cellular machinery into 19-22 nt siRNA, thereby suppressing the target gene expression. Though siRNA and shRNA elicit comparable results in RNAi experiments, the use of shRNA expression vectors is more appealing with several advantages over chemically synthesized siRNA. First, the use of plasmid to express shRNA is fairly inexpensive and has been shown to achieve long-term target gene suppression in cells and whole organisms. Second, the efficient delivery and stable integration of these shRNA expression cassettes into the host genome can be efficiently achieved by using various viral systems. Third, inducible or cell-specific gene silencing can be obtained in vivo by using a DNA-based shRNA vector. Fourth, vector-based RNAi can be used to rapidly generate knockdown/knockout mice, which would be useful models for unraveling the genetic roots of many human diseases. In the past few years, various groups, including our own, have developed systems for vector-mediated specific RNAi in mammalian cells. Regarding the construction of shRNA vector, the most common strategy requires the synthesis, annealing and ligation of two complementary oligonucleotides encoding a desired shRNA target sequence into an expression vector (32). The small DNA inserts prepared from the annealed oligonucleotides consist of 19-29 nt complimentary to the target sequence followed by its antisense sequence placed in the inverse orientation, separated by a spacer to make the hairpin loop. A terminal signal of 5-6 T and the corresponding overhangs for cloning are also included. Although this method is quick, it often suffers from mutation problems (32, 37). Typically, 20-50% of cloned shRNA constructs contain significant mutations as determined by DNA sequencing. The mutation frequency is close to 75% when the desired siRNA sequence is 29 nt in size (37). The unreliability of this method is in part due to the errors in the synthesis of long oligonucleotides (>50-mer). To verify the shRNA constructs that do not contain any errors, it was advised to pick up at least a few bacterial colonies for sequencing (38). Obviously, this process is time-consuming and costly. Another strategy that fewer people use in constructing shRNA vector is a PCR approach. With this approach, a promoter sequence serves as the template with an upstream primer that is complementary to the 5′ end of the promoter region, and a downstream primer containing the desired hairpin siRNA target sequence and a region that is complementary to the 3′ end of the promoter (22). Although it allows successful amplification of hairpin structures in a single amplification step, the correct amplicon production is critically dependent upon on the quality of downstream primer. For this reason, the method requires costly purification of the long downstream primer. shRNA expression vector can also be produced from target cDNA by enzymatic digestion (30). However, this method involves a multi-step process and may increase off-target effects. Recently, McIntyre and Fanning (31) reported an alternative approach to construct shRNA expression vector through the primer extension using a long template oligonucleotide and a short universal primer. The mutation rate was decreased by using DNA polymerase Phi29. However, the method still utilizes one long template oligonucleotide (72 nt if the siRNA sequence is 21 nt), which is not a trivial task. The strong secondary structure within this long oligonucleotide led to the difficulty of chain elongation. Kim et al. (25) described another approach of generating shRNA with short oligonucleotides. It is more cost-effective and less error-prone, but the shRNA vector coming from this method may be less potent because the loop sequences must be palindromic.
As can be seen, current methods of constructing shRNA vectors are costly and often suffer from mutation problem during synthesis.